Method for laying a pipeline having an inner corrosion proof cladding

ABSTRACT

Method for laying a pipeline onto a seabed from a lay barge, the pipeline having an inner corrosion proof metallic cladding that is closely fitted with metallic contact to an outer pipe material that is less corrosion proof, distinguished in that a) a section of the pipeline is reeled onto a pipe laying drum, whilst an overpressure of 5-25 bar is maintained within the section by means of a pressurised fluid inside the section, b) a further pipeline section is joined to the section already reeled onto the pipe laying drum, whilst the pipeline is motionless without mechanical movement, as the overpressure can be relieved as long as the sections are without mechanical movement, c) an overpressure of 5-25 bar is applied within the sections and the further section is reeled onto the pipe laying drum, d) several sections are joined together and reeled onto one or several pipe laying drums by repeating step b) and c) until the predetermined pipeline length is achieved, e) the pipeline is laid from the lay barge onto the seabed using conventional method, whilst an overpressure of 5-25 bar is maintained within the pipeline by means of a pressurised fluid until the pipeline is correctly placed onto the seabed.

AREA OF INVENTION

The current invention relates to pipelines, which are laid onto the seabed, and more precisely a method for laying a pipeline onto a seabed from a lay barge, the pipeline having an inner corrosion proof metallic cladding that is closely fitted with metallic contact to an outer pipe material that is less corrosion proof. With the method according to the invention, a pipeline of the type mentioned can be arranged onto and lay into place from a laying drum on the lay barge.

BACKGROUND OF THE INVENTION AND PRIOR ART

With an increasing need for transporting unprocessed well streams from underwater facilities, and more frequent occurrence of aggressive fluids with need to be transported, there is an increased demand for corrosion proof pipelines. However, the cost for pipelines of corrosion proof material has dramatically increased, and less expensive alternatives are therefore being sought after.

One such less expensive alternative is a pipeline of carbon steel with an inner more corrosion proof cladding. The strength of the carbon steel is thereby combined with the corrosion resistance of the cladding. The cladding is typically produced from a corrosion proof steel, a so-called stainless steel, and the cladding typically has a thickness from 1 to 7 mm, most typically about 3 mm. The cladding is fitted to the carbon steel pipeline's interior surface, either with mechanical contact or with metallurgical bonding by means of hot rolling (roll welding), welding, brazing or clad welding. The obviously least expensive alternative is that the inner cladding is fitted mechanically tightly against the carbon steel pipeline with metallic contact without any room in between. However, this embodiment has proven to be inadequate with respect to laying by reeling out from a pipe laying drum onboard a lay barge because the inner cladding becomes deformed by so-called “buckling/wrinkling”, such as bulging, buckling, and subsequently possible fracturing. Such deformation will result in that the inner cladding will loosen from the carbon steel pipeline's interior surface, causing it to loose its ability to resist the impact during operation of the pipeline. This represents a problem, which needs to be solved.

No prior art is known solving the above-mentioned problem, which previously appears not to be particularly focused upon. Background art is represented by the patent publications US 2003/0056954 A1, which relates to a method for maintaining a pipeline in operation; U.S. Pat. No. 6,142,707, which relates to direct electrical heating of a pipeline; and EP 1233143 A1, which relates to coiled tubing operations inside wells. Said patent publications describe an inner pipe installed within an outer pipe, but with spacing between said pipes and for most embodiments the inner pipe is a composite coiled tubing. The prior art referred to above relates to other problems than the problem underlying the present invention.

SUMMARY OF THE INVENTION

The above mentioned problem is solved with this invention by providing a method for laying a pipeline onto a seabed from a lay barge, the pipeline having an inner corrosion proof metallic cladding that is closely fitted with metallic contact to an outer pipe material that is less corrosion proof. The method is distinguished in that

-   a) a section of the pipeline is reeled onto a pipe laying drum,     whilst an overpressure of 5-25 bar is maintained within the section     by means of a pressurised fluid inside the section, -   b) a further pipeline section is joined to the section already     reeled onto the pipe laying drum, whilst the pipeline is motionless     without mechanical movement, as the overpressure can be relieved as     long as the sections are without mechanical movement, -   c) an overpressure of 5-25 bar is applied within the sections and     the further section is reeled onto the pipe laying drum, -   d) several sections are joined together and reeled onto one or     several pipe laying drums by repeating step b) and c) until the     predetermined pipeline length is achieved, -   e) the pipeline is laid from the lay barge onto the seabed using     conventional method, whilst an overpressure of 5-25 bar is     maintained within the pipeline by means of a pressurised fluid until     the pipeline is correctly placed onto the seabed.

There are particularly two features of importance in this connection, namely that it is adequate with a relatively moderate overpressure, typically 5-25 bar, in order to avoid deformation (buckling/wrinkling) and lack of contact between the inner cladding and the outer pipeline material, and that the pipeline or pipeline sections shall have no mechanical movement when the prescribed overpressure is not applied within the pipeline or sections thereof. The term “without mechanical movement” means no reeling onto or unwinding from the pipe laying drum; or lowering of the pipeline from the lay barge onto the seabed, without the prescribed overpressure. Strain for instance during welding of the pipe sections is acceptable, so is minor deformations, as long as the deformation is minor compared to what the pipeline and components thereof are exposed to when reeling onto a typical pipe laying drum. The term an overpressure of 5-25 bar, refers to that the inner cladding is biased with a pressure corresponding to 5-25 bar absolute (0.5-2.5 MPa) against the interior surface of the outer pipe material, which is less corrosion proof. A pressure lower than approximately 5 bar results in risk for deformation such as “buckling/wrinkling”, whilst a higher pressure than approximately 25 bar would be impractical and may create problems.

DETAILED DESCRIPTION

The laying of pipelines onto the seabed is normally operated by means of large specialised lay barges. By using large drums, so-called pipe laying drums, with a diameter typically between 10 and 30 meters, it is possible to even lay massive steel pipelines, which are reeled onto the pipe laying drum beforehand. The pipeline is then lowered onto the seabed using conventional method, for instance by S- or J-laying referring to the shape of the pipeline through the sea. The pipe laying drum typically has installed many pre-fabricated sections, where each section is typically 1 km long. The cladding is typically of stainless steel with a thickness of 3 mm, although other materials and thicknesses may also be usable. Increased cladding thickness may be an advantage as regards to pipes with large dimensions, in order to maintain correct surface pressure between the pipe materials. The exterior pipe material, which is less corrosion proof, is typically a thick walled carbon steel pipe, but other materials may also be usable.

So far it has not been possible to lay pipelines with an inner cladding of the type mentioned above since the cladding has been deformed by buckling, and this is the problem solved with the current invention.

With the current invention, an overpressure is applied within the pipeline or the sections thereof; at typically 5-25 bar by means of a pressurised fluid within the sections or pipeline. The necessary pressure is dependent on the diameter of the pipeline and the thickness of the inner corrosion proof cladding. Increased pressure will be necessary for increased diameter and a thinner cladding. A 5-10 bar overpressure is adequate for a 8″-10″ pipe and a 2-3 mm thick cladding, whilst 10-20 bar overpressure is adequate for a 16″ pipe with 2-3 mm thick cladding. The fluid can be nearly any form of liquid or gas, such as water under pressure, air (atmosphere) under pressure or an inert gas under pressure. On most occasions, water is preferred as the pressure medium because water is easily available and it is non-compressible and easy to pressurize to suitable pressure. If the vessel's capacity for storing and/or tension becomes critical due to the weight of the water-filled pipelines on the pipe laying drum, the preference is to use gas, for example compressed air. A necessary low overpressure will also make it more suitable to use a gas as pressurised fluid.

Preferably, the entire pipeline should be reeled onto one pipe laying drum to avoid joining between pipeline lengths on different pipe laying drums. The problem of joining may however be avoided by bringing the pipeline though the centre of one pipe laying drums to the centre of the next pipe laying drum, and rotate the drums synchronically during the laying and winding of the pipe. Alternatively, separate methods may be used for joining the pipeline sections, for instance by installing an ice plug on either one or both sides of the joint, whereby the overpressure can be maintained. Other devices, such as plugs and valves are also usable; likewise, various connecting subs and swivels.

Pumps, compressors, and necessary pipe system to control the overpressure need to be installed onboard the lay barge. The pipe sections are preferably mounted on to one or several pipe laying drums, directly onboard the pipe-laying vessel, otherwise the drums holding the pipe sections may be transported out to the lay barge and transferred onboard. 

1. Method for laying a pipeline onto a seabed from a lay barge, the pipeline having an inner corrosion proof metallic cladding that is closely fitted with metallic contact to an outer pipe material that is less corrosion proof, characterised in that a) a section of the pipeline is reeled onto a pipe laying drum, whilst an overpressure of 5-25 bar is maintained within the section by means of a pressurised fluid inside the section, b) a further pipeline section is joined to the section already reeled onto the pipe laying drum, whilst the pipeline is motionless without mechanical movement, as the overpressure can be relieved as long as the sections are without mechanical movement, c) an overpressure of 5-25 bar is applied within the sections and the further section is reeled onto the pipe laying drum, d) several sections are joined together and reeled onto one or several pipe laying drums by repeating step b) and c) until the predetermined pipeline length is achieved, e) the pipeline is laid from the lay barge onto the seabed using conventional method, whilst an overpressure of 5-25 bar is maintained within the pipeline by means of a pressurised fluid until the pipeline is correctly placed onto the seabed.
 2. Method in accordance with claim 1, characterised in that the fluid which is used inside the pipeline or sections thereof, is water.
 3. Method in accordance with claim 1, characterised in that the fluid which is used inside the pipeline or sections thereof, is compressed air.
 4. Method in accordance with claim 1, characterised in that the inner metal cladding is an approximately 3 mm thick cladding of stainless steel.
 5. Method in accordance with claim 1, characterised in that sections of the pipeline are welded together without an overpressure within the pipeline, as both the pipeline and sections are kept still with no mechanical movement.
 6. Method in accordance with claim 1, characterised in that increased diameter to the pipeline demands an increased overpressure in order to secure the integrity and configuration of the inner corrosive cladding. 